Method and apparatus for detecting particles in a flow

ABSTRACT

An apparatus for detecting particles in a flow comprises a probe  1  positioned so that it projects into the flow and is charged triboelectrically by the particles in the flow. An electric circuit is coupled to the probe  1  and includes evaluating means for monitoring a signal from the probe  1  and for providing an output in dependence on the signal generated by the triboelectric charging of the probe  1.  The part of the probe  1  that projects into the particle flow comprises an electrically conducting core covered with an insulating layer  2  which insulates the core from the particle flow.

This invention relates to a method and apparatus for monitoringparticles.

The invention particularly relates to an arrangement in which a probeprojecting into a flow of particles is charged triboelectrically byflowing particles colliding with the probe.

WO 86/02454 describes an apparatus for monitoring particles in a gasflow through a conduit. A metal probe is installed in a flow of gascontaining solid particles and the probe is coupled to an electriccircuit containing processing means. The probe is chargedtriboelectrically by the particles colliding with the probe and theresulting current in the circuit is processed to give an output thatgives a measure of the flow rate of the particles.

U.S. Pat. No. 5,054,325 shows an apparatus for measurement of fluidflows with suspended solid particles, using a triboelectric probeembedded in the wall of the conduit through which the fluid flows, wherethe fluid flow is a liquid or gas.

U.S. Pat. No. 5,591,895 the content of which is incorporated herein byreference also describes an arrangement for monitoring particles in agas flow. An electrically conducting rod is mounted in a stack and iscoupled to a processing circuit. The rod is charged triboelectrically bythe particles in the gas flow and the signal generated in the circuit isevaluated to give an output giving an indication of the particle flow.The rod is insulated at the point where it is mounted in the stack wallto prevent currents being transmitted to and from the stack wall, but ofcourse the insulating material does not extend over the whole outersurface of the conducting rod.

In the arrangements described above, a probe extends into the fluid flowand obstructs the flow of the particles. It has been found that if theprobe is mounted in a duct wall or the like, particles can build up inthe region between the electrically conducting probe and the duct walland, especially in damp conditions, form an electrically conducting pathbetween the probe and the duct wall.

As a result, the charge transferred to the probe by the particles in theflow can pass through the built-up material and through the duct wall toearth. Thus the output of the processing circuit may not give anaccurate measure of the particle flow.

In addition, small currents that exist in the duct wall can betransmitted via the probe into the processing circuit connected to theprobe. The magnitude of the currents generated by the triboelectricalcharging of the probe in the circuit may be of the same order as thosethat exist in the duct wall and so the output of the circuit may notgive an accurate measure of the particle flow.

A further problem is that the metal probe and the duct wall may begin tooperate as a battery, introducing more undesirable currents into theprocessing circuit, again giving an inaccurate measure of the particleflow.

It is an object of the invention to provide an improved method andapparatus for detecting particles in a flow that avoids or mitigates theabove problems and gives a reliable indication of the particle flow.

Accordingly, the present invention provides a method of detectingparticles in a flow in which a probe is positioned so that it projectsinto the flow of particles and is charged triboelectrically by theparticles in the flow and a signal from the probe is evaluated toprovide an indication of the particle flow, characterised in that thepart of the probe that projects into the particle flow comprises anelectrically conducting core covered with an insulating layer whichinsulates the core from the particle flow.

Advantageously the A.C. component of the signal is evaluated to providean indication of the particle flow. Although the A.c. component of thesignal generated in the circuit by triboelectrical charging of the probeis small when compared with the D.C. component, it has been found thatthe A.C. component of the signal gives a more accurate reflection of theparticle flow than the absolute value of the signal. It is believed thatfactors such as humidity, electrical charges already on the particlesand a build-up of particles on the probe all affect the absolute valueof the current without affecting the alternating component of thecurrent as much. We have found furthermore that the combination ofproviding an insulated probe and evaluating the A.C. component isespecially advantageous because the use of A.C. is especially suited tothe case where the probe is insulated.

Preferably the alternating component of the signal from the probe isfiltered to limit the frequency to below about 5 Hz. The frequency maybe limited to below 2 Hz, preferably about 1.5 Hz. By eliminating higherfrequencies the risk of spurious signals derived from mechanicalvibration of the probe is substantially reduced since the resonantfrequency of such vibration is likely to be substantially higher than 5Hz.

Preferably the alternating component of the signal from the probe isfiltered to limit the frequency of the signal to above about 0.1 Hz,preferably about 0.15 Hz. By eliminating lower frequencies the risk ofspurious signals derived from transient temperature-generated voltagesis substantially reduced.

Preferably the alternating component of the signal from the probe isamplified in a plurality of stages. In that case low frequencies, whichmay be those below 0.15 Hz, are preferably attenuated at the first stageof amplification.

The particles may be suspended in a fluid flow. The fluid flow may beeither a gas or a non-electrically conducting liquid and the particlesmay be either liquid or solid particles.

The flow may be a gas flow through a stack with suspended particles thatare emitted through the stack.

The flow may be through a duct having a probe mounted in the duct.

The present invention also provides an apparatus for detecting particlesin a flow comprising a probe to be positioned so that it projects intothe flow to be charged triboelectrically by the particles in the flow,and an electric circuit coupled to the probe having evaluating means formonitoring a signal from the probe and for providing an output independence on the signal generated by the triboelectric charging of theprobe, characterised in that the part of the probe to project into theparticle flow comprises an electrically conducting core covered with aninsulating layer which insulates the core from the particle flow.

Advantageously, the electric circuit comprises evaluating means formonitoring the A.C. component of the signal from the probe. The probemay be in the form of a rod. The rod probe may be of circularcross-section.

It is, of course, entirely unconventional to use an insulated probe tomonitor triboelectrical charging but we have found the use of such aprobe to be surprisingly effective in the present invention, especiallyin terms of overcoming the problems referred to above.

In the prior art particle monitoring arrangements, a conducting rodprobe is mounted in a duct wall with the electrically conducting surfaceof the probe exposed to the gas flow and the probe is coupled to aprocessing circuit. It is believed that a current is generated by therod probe in the following ways:

(1) When a particle collides with the probe there is a “rubbing” of theparticle against the probe leading to direct triboelectric charging.

(2) Particles in the flow may become charged by collisions with otherparticles. When a charged particle collides with the conducting probe,the particle gives up some or all of its charge to the probe. Theparticle may be charged positively or negatively and the currentgenerated will vary accordingly.

(3) A charged particle in the flow passing the probe may, even though itdoes not touch the probe, induce a charge in the probe which causes acurrent to flow.

In the case of the present invention it is believed that currents areusually generated as a result of all three of the effects describedabove although precisely what happens is not fully understood. It isbelieved that the probe and evaluating means of the present inventionmay be likened in electrical terms to the same evaluating means coupledto a non-insulated probe connected in series with a capacitor.

Advantageously,the size and composition of the particles in the flowdoes not vary and the flow is monitored in order to detect variations inthe mass flow rate. Given that the size of the particles and theircomposition does not vary, the measurement of mass flow rate canalternatively be regarded as a measurement of the flow rate in terms ofthe number of particles per unit time.

Usually it will be desired to provide a quantitative indication of themass flow rate but for some applications it may be adequate simply toprovide an indication of whether or not the mass flow rate measured isabove or below some threshold level. An alarm may be sounded if the massflow rate is above the threshold level.

The invention can be used to monitor a flow of solid particles alone orto monitor solid or liquid particles suspended in a gas or liquid flow.The invention can be used to provide a continuous measurement of themass flow rate of the suspended particles. The invention has manyapplications in industrial plants using particle collection and drysolids handling processes. It may be used, for example, to monitor theperformance of a filter. A particularly advantageous and important usefor the invention is the measurement of the emission of particlesclassified as pollutants through a stack to the atmosphere. Theinvention can also be used in a manufacturing process where it isnecessary to monitor and control the addition or recovery of particulatematter. For example, the invention may be used in a system whereparticles are suspended in a gas stream, as in a pneumatic conveyingsystem.

An apparatus and method for monitoring flow of particles in accordancewith the invention will now be described by way of example only withreference to the accompanying drawings, in which:

FIG. 1 is a sectional view of the sensing head of the dust flowmonitoring apparatus mounted in the wall of a stack through which dustparticles in a flow of air are emitted;

FIG. 2 is a sectional view of the insulated probe assembly of thesensing head;

FIG. 3 is a block diagram representation of the electrical system of thedust flow monitoring system;

FIG. 4 is a graph showing the d.c. voltage outputs from the monitoringsystem monitoring an air flow to which polyvinylchloride (PVC) dustparticles have been added at two different constant rates.

With reference to FIG. 1 of the accompanying drawings, the sensing headof the dust flow monitoring apparatus generally comprises a metal probe1 with a layer of insulating material 2 on the outer surface, aninsulated support member 3, made of PTFE, and an electronic sensor unit4 comprising a waterproof box 5 containing an electronic circuit board6.

With reference to FIG. 2, the probe assembly is a metal rod 1 ofcircular cross-section with a diameter of 16 mm and 150 mm long made ofstainless steel and coated on the outer surface, using known techniques,with a nylon powder coating 2. The stainless steel rod is coated withthe insulating material over the entire surface except the end of therod that is to be connected to the circuit board. The thickness of theinsulating layer is 5 μm.

FIG. 1 shows the probe and a sensor unit 4 making up the sensing headfitted in the stack wall 8.

The electronic sensor unit 4 comprises an aluminium cover 5 containing acircuit board 6 carrying signal evaluating means and is connected to themetal probe by means of a connection screw 7. The connection screw 7passes through the circuit board 6 and connects it to the metal probe 1.

The sensing head is mounted in the stack wall 8 in the following way. Aposition for mounting the sensing head 11 in the stack is chosen wherethe gas flow is reasonably linear, for example, as shown, in a straightsection of the stack. An internally screw threaded sleeve 9 is thenwelded into the stack at the chosen position. An outer metal sleeve 10having an external screw thread matching the internal thread on thecoupling is fitted around the insulating support member 3. The sensinghead is then inserted into the stack wall and screwed into place so thatthe probe extends into the gas flow. A lock nut 11 secures the sensinghead in the stack wall. A second lock nut 12 acts on a sealing gasket 13that is placed around the end of the probe between the metal sleeve 10and the sensing unit 4.

As shown in FIG. 1 the probe projects into the shaft of the stack in adirection transverse to the direction of flow of air through the stack.Particles in the air flow collide with the probe and triboelectriccharging takes place. A signal is thus produced in the circuit coupledto the probe.

FIG. 3 shows a block diagram illustrating the electronic circuitry ofthe dust monitoring apparatus. Our co-pending application no. GB90.09407 describes in full detail the circuitry used to evaluate acurrent generated by a metal probe in an apparatus for detectingparticles in a gas flow. The same circuitry is used in the dustmonitoring apparatus described here. However, as described above, thesignal generated in the circuit by the insulated probe is different fromthe signal generated in the circuit when it is coupled to anon-insulated metal probe. The electronic circuit board in the sensorshown in FIG. 1 contains the input amplifier 22, the first couplingnetwork 23, the second stage amplifier 24 and the gain-change logiccircuit 25. The rest of the electrical system is “control room”equipment and is located at a position remote from the sensing head.

The signal is processed in the same way as is described in ourco-pending application GB 2 266 772. Briefly, referring to FIG. 3, thecurrent supplied by the probe is amplified and subjected to bandwidthshaping. The signal is also passed through coupling networks 23,102containing capacitors that block the d.c. and very low frequency signalsin the circuit. Finally, the signal is passed to an averaging filter andoutput amplifier 104 that provides a long-term average of the signals,reducing the random signal variations which particle flow provides. Theoutput signal from the output amplifier is passed to avoltage-to-current converter 105 for driving a pen-recorder or the like.

Also, a signal from the averaging filter and output amplifier 104 isapplied to the alarm logic and controller 106 which is set to triggerwhen a set level is exceeded. There is also an arrangement for settingthe alarm logic and controller to trigger when the applied signal fallsbelow a set threshold.

Because the outer layer of the probe is electrically insulated, even ifthere is a build-up of particles in the region between the probe and thestack wall, there is no electrically conducting path between the stackwall and the processing circuit.

In that way the reading given by the dust monitoring apparatus is a moreaccurate measure of the particle flow.

FIG. 4 shows the results, in the form of voltage in volts plottedagainst time in minutes, of a test carried out to measure the result ofmonitoring particles using the monitoring apparatus described above atgiven densities in an air flow through a duct. In the test, PVCparticles of substantially constant size of 200 microns were added toair flowing at a rate of 18.5 m/s through a duct of circularcross-section. The particles were first added at a substantiallyconstant rate of the order of 7.7 mg/m3 and the output from theevaluating means was measured. Similarly, PVC particles were later addedat a substantially constant rate of approximately 3.1 mg/m3 and theoutput was again measured. FIG. 4 shows that the reading obtained by theevaluating means in each case is substantially constant andapproximately proportional to the density of the particles.

The invention is not limited to use in a stack as described above. Aswill be clear to a person skilled in the art, the invention can be usedto monitor any flow of particles, whether flowing under the action ofgravity, or suspended in a gas or non-electrically conducting liquid.Other types of insulating material would be suitable for use on theprobe. For example, the insulating layer could be nylon,polytetrafluoroethene (PTFE), ceramic or any plastic, whetherthermoplastic or thermosetting, and it could be in the form of a coatingor a sleeve. Although the circuit described above coupled to the probeevaluates the A.C. component of the signal generated by the probe, theinsulated probe is also suitable for use with a circuit where the D.C.component, or both the A.C. and the D.C. component, of a signalgenerated by the probe is evaluated to give an output reading that is ameasure of the particle flow.

What is claimed is:
 1. A method of detecting particles flowing along aemitted through the stack in which a probe is positioned so that itprojects into the flow of particles and is charged by the particles inthe flow characterized in that the part of the probe that projects intothe particle flow comprises an electrically conducting core covered withan insulating layer which insulates the core from the particle flow andthe A.C. component of a signal from the probe is evaluated to provide anindication of the particle flow.
 2. Method as claimed in claim 1 inwhich the particles are suspended in a fluid flow and the probe ischarged triboelectrically by the particles in the fluid flow.
 3. Amethod as claimed in claim 1 in which the fluid is a gas and theparticles are liquid or solid particles suspended in the gas.
 4. Amethod as claimed in claim 1 in which the A.C. component of the signalfrom the probe is filtered to exclude high frequency components of thesignal.
 5. A method as claimed in claim 4 in which the A.C. component ofthe signal from the probe is filtered to limit the frequency to belowabout 5 Hz.
 6. A method as claimed in claim 1 or claim 4 in which theA.C. component of the signal is filtered to exclude low frequencycomponents of the signal.
 7. A method as claimed in claim 6 in which theA.C. component of the signal is filtered to limit the frequency to aboveabout 0.1 Hz.
 8. Apparatus for detecting particles flowing along a stackand emitted through the stack comprising a probe to be positioned sothat it projects into the flow to be charged by the particles in theflow, and an electric circuit coupled to the probe characterized in thatthe part of the probe to project into the particle flow comprises anelectrically conducting core covered with an insulating layer whichinsulates the core from the particle flow, and the electric circuit hasevaluating means for monitoring an A.C. component of the signal from theprobe for providing an output in dependence on the signal generated bythe triboelectric charging of the probe.
 9. Apparatus as claimed inclaim 8 characterized in that the probe is in the form of a rod. 10.Apparatus as claimed in claim 9 characterized in that the rod is ofcircular cross-section.
 11. Apparatus as claimed in claim 8 in which theelectric circuit comprises filter means for filtering out high frequencycomponents of the signal.
 12. Apparatus as claimed in claim 11 in whichthe filter means are for limiting the frequency of the A.C. component ofthe signal to below about 5 Hz.
 13. Apparatus as claimed in claim 8 orclaim 11 in which the electric circuit comprises filter means forfiltering out low frequency components of the signal.
 14. Apparatus asclaimed in claim 13 in which the filter means are for limiting thefrequency of the A.C. component to above about 0.1 Hz.
 15. A method ofdetecting particles flowing along a stack and emitted through the stackin which a probe is positioned so that it projects into the flow ofparticles and is charged by the particles in the flow characterized inthat the part of the probe that projects into the particle flowcomprises an electrically conducting core covered with an insulatinglayer which insulates the core from the particle flow, the signal fromthe probe is filtered to block the D.C. signal and the A.C. signal isevaluated to provide an indication of the particle flow.
 16. Apparatusfor detecting particles flowing along a stack and emitted through thestack comprising a probe to be positioned so that it projects into theflow to be charged by the particles in the flow, and an electric circuitcoupled to the probe characterized in that the part of the probe toproject into the particle flow comprises an electrically conducting corecovered with an insulating layer which insulates the core from theparticle flow, and the electric circuit has filter means for blockingthe D.C. signal from the probe an evaluating means for monitoring thefiltered A.C. signal for providing an output in dependence on the signalgenerated by the triboelectric charging of the probe.
 17. A method ofdetecting particles flowing along a stack and emitted through the stackin which a probe is positioned so that it projects into the flow ofparticles and is charged by the particles in the flow characterized inthat the part of the probe that projects into the particle flowcomprises an electrically conducting core covered with an insulatinglayer which insulates the core from the particle flow, that the signalfrom the probe is filtered to limit the signal to an A.C. signal offrequency above about 0.1 H_(z) and the A.C. signal is evaluated toprovide an indication of the particle flow.
 18. Apparatus for detectingparticles flowing along a stack and emitted through the stack comprisinga probe to be positioned so that it projects into the flow to be chargedby the particles in the flow, and an electric circuit coupled to theprobe characterized in that the part of the probe to project into theparticle flow comprises an electrically conducting core covered with aninsulating layer which insulates the core from the particle flow, andthe electric circuit has filter means for limiting the signal from theprobe to an A.C. signal of frequency above about 0.1 H_(z) andevaluating means for monitoring the filtered A.C. signal for providingan output in dependence on the signal generated by the triboelectriccharging of the probe.
 19. A method of detecting particles flowing alonga stack and emitted through the stack in which a probe is positioned sothat it projects into the flow of particles and is charged by theparticles in the flow characterised in that the part of the probe thatprojects into the particle flow comprises an electrically conductingcore covered with an insulating layer which insulates the core from theparticle flow and a signal from the probe is filtered to excludefrequency components below about 0.1 Hz and above about 5 Hz and isevaluated to provide an indication of the particle flow.
 20. A method ofdetecting particles flowing along a stack and emitted through the stackin which a probe is positioned so that it projects into the flow ofparticles and is charged by the particles in the flow characterised inthat the part of the probe that projects into the particle flowcomprises an electrically conducting core covered with an insulatinglayer which insulates the core from the particle flow, the insulatinglayer being substantially thinner than the conducting core, and a signalfrom the probe is evaluated to provide an indication of particle flow.21. A method as claimed in claim 20, in which the signal for the probeis filtered to exclude frequency components below about 0.1 Hz.
 22. Amethod as claimed in claim 20, in which the signal for the probe is acurrent signal which is converted into a voltage signal, said voltagesignal being passed through signal processing means which block the DCcomponent of the voltage signal to generate a processed voltage signaland said processed voltage signal being subsequently evaluated toprovide an indication of the particle flow.
 23. A method of detectingparticles flowing along a stack and emitted through the stack in which aprobe is positioned so that it projects into the flow of particles andis charged by the particles in the flow characterised in that the partof the probe that projects into the particle flow comprises anelectrically conducting core covered with an insulating layer whichinsulates the core from the particle flow and the probe generates acurrent signal which is converted into a voltage signal, said voltagesignal being passed through signal processing means which block the DCcomponent of the voltage signal to generate a processed voltage signal,and, said processed voltage signal being subsequently evaluated toprovide an indication of the particle flow.
 24. A method as claimed inclaim 23, in which said processed voltage signal excludes frequencycomponents below about 0.1 Hz.
 25. Apparatus for detecting particlessuspended in a gas flow flowing along a gas flow path emitted through asack, the apparatus comprising: a probe projecting into the gas flowpath for detecting particles in the gas flow along the stack, a walldefining a boundary of the gas flow path, the probe comprising anelectrically conducting core and a covering layer, the covering layerbeing electrically insulating and serving to block conduction of a DCcurrent from the surface of the probe through the insulating layer tothe electrically conducting core of the probe, the probe being fixed inthe wall with the electrically conducting core electrically insulatedfrom the wall, and an electric circuit connected to the electricallyconducting core of the probe for evaluating the signal from the core ofthe probe for providing an output in dependence on the signal generatedby the charging of the probe.
 26. Apparatus as claimed in claim 25, inwhich the probe is in the form of a rod.
 27. Apparatus as claimed inclaim 26, in which the rod is of circular cross-section.
 28. Apparatusas claimed in claim 25, in which the electric circuit comprises filtermeans for filtering out high frequency components of the signal. 29.Apparatus as claimed in claim 28, in which the filter means are forlimiting the frequency the AC component of the signal to below about 5Hz.
 30. Apparatus as claimed in claim 25, in which the electric circuitcomprises filter means for iltering out low frequency components of thesignal.
 31. Apparatus as claimed in claim 30, in which the filter meansare for limiting the frequency of the AC component to about 0.1 Hz. 32.A method for detecting particles flowing in a gas flow along a stack andemitted through the stack in which a probe including a portion whichcomprises an electrically conducting core covered by an insulating layerwhich insulates the core from particle flow is positioned so that saidportion projects into the flow of particles in the stack and is chargedtriboelectrically by particles in the flow and the quantities ofelectrical charges transferred to the probe are evaluated to provide anindication of the particle flow in the gas flow, wherein, in order toreduce the effect of variations in gas flow related variables other thanthose relating to particle flow, an alternating component in the signalcaused by the triboelectrical charging of the probe is monitored, thealternating component of the signal from the probe is filtered toexclude high frequency components of the signal and the magnitude of theresidual alternating component is itself used to give an indication ofthe particle flow through the stack.
 33. The method according to claim32 in which the alternating component of the signal from the probe isfiltered to limit the frequency to about 0.10 Hz.
 34. The methodaccording to claim 32 in which said insulating layer is substantiallythinner than said conducting core.